The present invention relates to underground gasification of brown and bituminous coals and may be used in gasification of coal seams under various bedding conditions (at different pitch angles and depths).
The invention may be used to best advantage with coal seams thicker than 5 m.
It may also find applications in underground gasification of oil shales.
With prior art methods of underground gasification of coals, a coal bed is degasified simultaneously throughout its thickness.
However, when a coal bed is degasified simultaneously throughout its thickness in fields characterized by a small ratio of the thickness of the degasified bed, there are formed cracks extending to the surface of the earth due to rock deformation and shifts. Thus, a gaseous medium supplied to effect gasification and the gas formed therewith break through said cracks and fissures.
A known method of underground gasification of coals (cf. U.S. Pat. No. 4,018,481, Cl. 299-4 published in 1977) comprises the steps of opening a coal seam by drilling wells throughout its thickness, interconnecting the wells through a seam hydrofracture, establishing a gasification passage and gasifying the coal by introducing a gaseous medium into a blast passage and removing the produced gas. A disadvantage of the aforesaid method is that, in gasification of thick coal seams deposited near the surface of the earth, there may occur downfalls or the blast and gas may break through cracks and fissures, thereby causing the gasification process to stop.
There is also known a method of underground gasification of coal (cf. U.S. Pat. No. 4,069,867, Cl. 166-256 published in 1978) comprising the steps of opening a coal seam by drilling vertical and inclined holes, interconnecting the holes within the coal seam, establishing gasification passages therein, and gasifying the coal by injecting a gaseous medium into one of the interconnected holes and removing the gas from another hole. A disadvantage of the foregoing method is that, in gasification of thick coal seams deposited at a small depth from the surface of the earth, there may occur downfalls or the blast and gas may break through to the surface of the earth whereby the gasification process is disrupted or stopped.